System of liquid electrodes for pockels cells and liquid compositions for said electrodes

ABSTRACT

A Pockels cell with liquid electrodes, in which said electrodes comprise a water solution formed by the salt of the electrooptical crystal and by another simple salt having an ion common with that of the salt of the electro-optical crystal.

United States Patent Berne et al.

[ 1 June 6, 1972 [54] SYSTEM OF LIQUID ELECTRODES FOR POCKELS CELLS ANDLIQUID COMPOSITIONS FOR SAID ELECTRODES [72] Inventors: Adriano Berne;Mario Cesarotti; Glauco Benedetti Michalagelli, all of Rome, Italy [73]Assignee: Selenia Industrie Elettroniche Associats S.p.A., Rome, Italy[22] Filed: Aug. 6, 1969 [21] Appl.No.: 848,046

[30] Foreign Application Priority Data Aug. 31, 1968 Italy .3944? N68U.S. Cl ..330/4.3, 350/ l 50 ..H01S 3/10 Field of Search ..350/150, 160,161, 267;

[56] References Cited UNITED STATES PATENTS 3,393,956 7/1968 Clark..350/ 150 FOREIGN PATENTS OR APPLICATIONS 1,173,033 12/1969 GreatBritain ..350/I50 Primary Examiner-Rodney D. Bennett, Jr.

Assistant Examiner-N. Moskowitz Attorney-Wenderoth, Lind 8L Ponack [57]ABSTRACT A Pockels cell with liquid electrodes, in which said electrodescomprise a water solution formed by the salt of the electro-opticalcrystal and by another simple salt having an ion common with that of thesalt of the electro-optical crystal.

6 Claims, 3 Drawing figures PATENTEDJuu 6 I972 SHEET 10F 2 INVENTORS MIE E m NWM R l EMU m E Awm I mAm AM U PATENTEUJun 6 I972 F 3TRANSHITTANCE FOR 1cm THlCKNESS Assommow COEFFICIENT SHEET 2 OF 2 'OOZELOOBOL y 00901 339w 'PN OOOOL yezse sasvw N- H OOZS YSILS 335V"! NOQHVSYSTEM OF LIQUID ELECTRODES FOR POCKELS CELLS AND LIQUID COMPOSITIONSFOR SAID ELECTRODES The present invention relates to a system of liquidelectrodes for Pockels cells and to the liquid composition for saidelectrodes.

In the embodiment of the Pockels cells exploiting the longitudinalelectro-optical effect, the greatest problem to be solved is thatconcerning the electrodes to be used for generating the electric field.

The quality of a Pockels cell is evaluated on the bases of a highoptical transmittance, a good uniformity of electrical field such as toensure, when the cell is energized, the complete extinction of theluminous beam passing through it, and a low electric loss. Said featuresdepend at the utmost upon the structure of the electrodes of the cell,which electrodes in the case of the longitudinal electro-optical effect,are also passed through by the light beam passing through the cell.

Various solutions have been found for the embodiment of said electrodes,like the net electrodes, the annular electrodes, the conductive glasselectrodes and the transparent layer electrodes, but none of theseembodiments proved to be entirely satisfactory.

The net electrodes, even if ensuring a good uniformity of the electricfield, have a high optical absorption (of the 30-40 percent order)associated to diffraction phenomena.

The annular electrodes, on the other hand, have the advantage of havingthe maximum optical transparency, but do not allow the obtaining of auniform electric field. Furthermore, for a good operation of the cell,it is required that the ratio between the distance between theelectrodes and the aperture of the electrode itself be 1.2, requiringthus great thicknesses of electro-optical materials when the Pockelscells must be used with light beams having large cross dimensions.Moreover said electrodes, due to the high lack of homogeneity of thefield, require bias voltages greater than those required by theelectro-optical materials themselves.

Little is known about the conductive glass electrodes; it is only knownthat they have a rather high coefficient of optical absorption,amounting to about 40 percent, a remarkable lack of homogeneity ofcomposition and a high resistivity.

The deposited transparent electrodes are those which until now haveafforded the best results. Said electrodes consist of very thin layersof conductive material deposited on quartz plates cemented to theelectro-optical materials with low resistivity cements. Said electrodesproduce a very uniform field, their resistance is rather low (of theorder of some hundreds or thousands of ohm square) whereby theirelectric loss is contained within non-excessive values for not very highoperative frequencies, and have a good optical transmittance which forthe best specimens is greater than 95 percent. With said electrodes itis also possible to use thin crystals (of the order of a fewmillimeters) for remarkable crosswise dimensions. But these optimumfeatures reached by the last described electrodes, are not sufficientwhen it is necessary to operate at high frequencies or it will benecessary to use several cells in serial sequence (as in the case of thedigital deflectors) as in the first case the electric losses and in thesecond the optical absorptions will have a pre-eminent importance.

Another solution for transparent electrodes consists in the use ofliquid electrodes, having a high electric conductivity and which allowthe advantage of a high transparency to be coupled to a low loss and toa high field uniformity.

The electro-optical crystals are generally highly soluble and can beeasily attacked. It is impossible to use protective layers on thecrystal as said layers having, in that they are highly transparent, ahigh resistivity, would introduce in the system high capacities andresistances in series, which would lead to remarkable increases of thefield necessary to the rotation of the half wave polarization plane,apart from the transient phenomena.

According to the present invention liquids for electrodes forelectro-optical crystals are provided, having features as follows: highelectric conductivity; high composition stability in the presence of thecrystal, also for remarkable thermal excursions; physical and chemicalcompatibility with the crystal; high optical transparency; a refractiveindex very near to that of the crystal; high response speed to theelectrical pulses, and finally inalterability of physical and chemicalstructure in the presence of high intensity and coherence light beams.

Said liquids consist of a water solution formed by the salt of theelectro-optical crystal and a simple salt having an ion in common and asolubility index slightly variable with the temperature. In order tobring the refractive index of said solution as near as possible to thatof the used electro-optical crystal, said solution will be then mixedwith a highly transparent liquid having a high refractive index andbeing infinitely soluble in water. The contents of said liquid in thesolution is a maximum in order to have a high electric conductivity thatis slowly variable when the concentration changes.

Various Pockels cells with liquid electrodes according to this inventionhave been embodied using as electro-optical material KDP and ADPcrystals.

The principles subsequently set out and the obtained results can be,however, easily generalized and extrapolated for all otherelectro-optical crystals.

LIQUID ELECTRODE POCKELS CELLS The features which substantiallydifferentiate the Pockels cell with liquid electrodes according to thisinvention, from a common cell are as follows:

I. The thickness of the electro-optical crystal can be limited to theminimum indispensable value due to manufacture. In fact the liquidelectrode allows the operation under electric fields greater than thebreakdown limits of the crystal itself. Furthermore no problem ofhomogeneity of the electric field exists, as the electrodes formed byliquids perfectly mate the shape of the surface. This last cited featureis common with the cells with deposited electrodes.

2. The liquid electrodes allow the use of crystals with a coarse surfacemanufacture. In fact in the cells where the electrodes do not adhere tothe crystal, each variation of the thickness of the latter originates avariation of the optical path, the electric field remaining constant. Inthe cells with deposited electrodes, the deposition is difficult if thesurface has a slight degree of finishing. In both cases any deformationof the surfaces, be it macroscopical (curvature) or microscopical(roughness) originates a deformation of the issuing wave which isproportional to the difference in refractive index existing between thecrystal and the surrounding medium (usually air). In the case of liquidelectrodes this sudden difference is extremely reduced.

If the cell requires closure glasses, it will be sufficient that onlythe outer surfaces thereof be optically worked out. This moves theproblem of the manufacture of the surfaces from the crystal (hardlyworkable, expensive and brittle) to the glass (easily workable and notexpensive).

3. The liquid electrodes as aforesaid, allow the embodiment of cellshaving a transmittance remarkably high with respect to the usual cells.In said conventional cells, use is made of either transparent conductiveelectrodes (with a high absorption) or apertured electrodes renderingnecessary high thicknesses of electro-optical material for obtainingsufficiently uniform fields. The advantage of the high transmittivity isparticularly remarked in electro-optical systems such as for instancethe digital deflection systems using several cells in series, and thelaser systems using one Pockels cell for the Q- switch. In the lattersystems each absorbing element, beyond causing a remarkable Q reductionof the resonant cavity, is easily destroyed by heating by the laser beamitself. In the arrangement according to this invention the necessaryvery thin layer of liquid (which can be less than 0.1 mm) has a hightransparency throughout the visible spectrum (see FIG. 3) for the caseof the solution a), and a refractive index sensibly close to the averageindex of the electro-optical crystal which reduces the losses due toreflection at the passages liquidcrystal-liquid, thus limiting thelosses practically within the absorption values of the crystal 8 percentfor 3 mm).

FIGS. 1 and 2 are generally representative of a Pockels cell in whichthe electrode of the present invention are used. FIG. 1 is a sectionalview along the plane A-A of FIG. 2, the latter being a front view of thecell. In FIGS. 1 and 2, the element 1 is the electro-optical crystal,the elements 2 are optically transparent plates, the elements 3 are thewire electrodes, 4 is the sealing cement and the elements 5 are the thinlayers of the filling liquid of the invention described above.

TEST RESULTS The liquid-electrode Pockels cells according to thisinvention have been experimentally embodied as follows:

One KDP crystal and one ADP crystal, dimensioned 2.5X2.5 cm with anoptical axis perpendicular to the 2.5 2.5 cm faces, are flattened andpolished using dry abrasive cloths (both KDP and ADP are highly solublein water).

The final surfaces seem polished, but when carefully exoptimal solutionwas so composed: per 100 cc of water use was made of 36 gms of Nl-LCIand 24 gms of ADP. This liquid has been mixed with glycerin used as thehigh refractive-index transparent liquid, in the amount of one part ofsolution per four parts of glycerin.

c. A saturated solution of KDP, KCl, ADP and NH Cl in water: the optimalsolution was so obtained: per 100 cc of water, use was made of 24 gms ofKCl, 16 gms of KDP, 36 gms of NH CI and 24 gms of ADP. The liquid hasbeen then mixed with glycerin, used as the high refractive-indextransparent liquid in the amount of one part of solution per four partsof glycerin. The obtained liquids had the features shown in the Table:

amined, a remarkable roughness is still visible. To the so workedcrystals two plates of optically worked quartz are adhered by means ofsilicone glue on the interposition along the edge of the crystal of a0.1 mm wire of silvered copper, utilized both as spacer for allowing theintroduction of the liquid, and as electric connection; the cells arefilled with the various liquids hereinafter described [the KDP with thevarious liquids a and c and the ADP with the various liquids of b andc], then sealed and submitted to the following tests:

a. Test of stability of operation in times.

During the tests lasting several days an optimum reproducibility of thevalue of the potential of M2 for determined wave lengths was found.

b. Tests in laser cavity.

The cell has been placed in various ruby and neodymium laser glasscavities. Single giant pulses have been obtained reproducible inamplitude and duration. Also under powers emitted by the laser in theorder of lOjoule (peaks of 260 MW having a sec duration) the cellappears not to have been damaged to any measure.

c. Non-linear effects.

Measures effected in the liquid revealed the existance of no appreciableRaman effect. (1. The cell has been used as a shutter between twocrossed polarizers. Energized with electric pulses with rise times ofabout 7 nano sec. the form of the output luminous signal result wasequal to that of the electric signal itself.

From the above appear very clearly the advantages of this Pockels cellwith liquid electrodes, with respect to the other types, said advantagesbeing summarized as follows:

a high stability with temperature a high optical and field homogeneity alow resistivity, with the consequent low electrical losses even underhigh repetition frequency operation speed of response low optical lossesnegligible non linear effects easy of manufacture and low cost.

The liquid a) serves as an electrode for KDP or KD*P crystals, theliquid b) for ADP crystals and the liquid c) for KDP. KD"P and ADPcrystals.

All features described were never found simultaneously in the previouslyembodied Pockels cells.

Said cells can be also used as amplitude modulators, connecting severalcells in series for lowering the half wave potential, this beingfeasible without inconveniences in view of the high transparency of thecells.

Having thus described the present invention, what is claimed is:

l. A Pockels cell with liquid electrodes, characterized in that saidelectrodes comprise a water solution formed by the salt of theelectro-optical crystal and by another simple salt having an ion commonwith that of the salt of the electro-optical crystal.

2. A pockels cell as claimed in claim 1, wherein said second additivesalt is selected among the salts having a solubility index slightlyvariable with the temperature.

3. A Pockels cell as claimed in claim 1, wherein said solu tion is mixedwith a highly transparent liquid having a high refractive index andsoluble in water.

4. A Pockels cell as claimed in claim 1, wherein the electroopticalcrystal consists of KDP (monobasic potassium phosphate) or KDP (markedmonobasic potassium phosphate) and said electrodes are made of saturatedsolutions of KDP and KCl in water mixed with glycerin.

5. A Pockels cell as claimed in claim 1, wherein the electroopticalcrystal consists of ADP (monobasic ammonium phosphate) and saidelectrodes consists of saturated solutions of ADP and NH C1 in water towhich glycerin has been added.

6. A Pockels cell as claimed in claim 1, wherein the electroopticalcrystal consists of KDP or KDP or ADP and said electrodes consist ofsaturated solutions of KDP, KC 1, ADP, and NH Cl in water to whichglycerin has been added.

2. A pockels cell as claimed in claim 1, wherein said second additivesalt is selected among the salts having a solubility index slightlyvariable with the temperature.
 3. A Pockels cell as claimed in claim 1,wherein said solution is mixed with a highly transparent liquid having ahigh refractive index and soluble in water.
 4. A Pockels cell as claimedin claim 1, wherein the electro-optical crystal consists of KDP(monobasic potassium phosphate) or KDXP (marked monobasic potassiumphosphate) and said electrodes are made of saturated solutions of KDPand KC1 in water mixed with glycerin.
 5. A Pockels cell as claimed inclaim 1, wherein the electro-optical crystal consists of ADP (monobasicammonium phosphate) and said electrodes consists of saturated solutionsof ADP and NH4C1 in water to which glycerin has been added.
 6. A Pockelscell as claimed in claim 1, wherein the electro-optical crystal consistsof KDP or KDXP or ADP and said electrodes consist of saturated solutionsof KDP, KC1, ADP, and NH4C1 in water to which glycerin has been added.